As well known, in the paper converting industry a variety of types is used of machines and of processes for making paper tissues, paper towels and similar articles in stacks of a certain height of interfolded sheets.
They are obtained stacking the sheets in an “interfolded” way, i.e. at each fold a wing of the previous sheet and a wing of a next stack sheet engage with each other. This way, when drawing a sheet from a package, at the moment of the use, also a wing of a next stack sheet is dragged up to protruding from the package, with subsequent practical employment for certain types of users. Among possible interfolding ways the L-type, with 2 panels (single fold), or the Z or W types, respectively with 3 and 4 panels (multi fold), are known.
The interfolding machines use one or more webs of paper, which from one or more large rollers and which are cut into sheets and then supplied offset with respect to one another on folding counter-rotating rollers.
More precisely, the webs are cut into sheets by means of cutting rollers that interact alternatively, with relative counter-support blades. In case of L-type interfolding (single-fold) the webs are cut to form a shifted succession of sheets coming preferably from two different directions. Then, the sheets coming from either directions are fed in an alternated way to the folding rollers, so that each sheet coming from the first direction is overlapped with a portion of the sheet coming from the second direction, and vice-versa.
The sheets coming from the two directions, in order to be overlapped in the way above described, adhere to the respective folding rollers by a holding system comprising either suction holes or mechanical clamps. Then, the downstream portion of each sheet leaves a respective folding roller at the contact line between the two rollers, held by the other folding roller, which is holding already the upstream portion of the previous sheet.
In case of Z or W type interfolding, or even in case of much more folds, as disclosed in U.S. Pat. No. 3,490,762, so-called “multifold”, the interfolding method can be similar to what described above, with the difference that The overlapping step between two successive sheets is carried out immediately after the cut and a stream of partially overlapped sheets reaches the folding rollers from a single direction.
In particular, the folding rollers have a circumference as a multiple to the length of the not overlapped portion of two overlapped sheets. Therefore, the stack of sheets adds an interfolded sheet after each fraction of turn of the folding rollers. This parameter determines the size of the folded sheet being stacked, i.e. the width of the interfolded sheets packages. In view of that, one of the parameters of reference for an interfolding machine is the stack width.
Another reference parameter for an interfolding machine is the length of the sheets, also called cut-off. In particular, in the interfolding machines the length of the interfolded sheets that eventually form the stack of final product is responsive to the circumference of the cutting rollers and to the angular distance among the cutting blades. In other words, the cutting length is fixed and is determined univocally by the circumference of the cutting roller or rollers.
By changing the length of the sheets, or cut-off, it is possible to keep the same pack width, by adjusting the number of interfolded panels.
It can be in particular preferable to adjust the cut-off without changing the pack width, leaving the user a variety of choices for making packs.
A common interfolding machine of “multifold” type allows to produce interfolded sheets of a single length, with an extremely stiff process, and for each sheet length a different machine is required.
In EP 1826165, in the name of the same applicant, an interfolding machine is described multi-fold of modular type, in which it is possible to cut a web of paper into sheets of different length after replacing a modular portion comprising the cutting roller and the transfer roller with another modular portion comprising a cutting roller and a transferring roller of different diameter, and then capable of cutting the web into sheets of different length and causing the sheets to be transferred to an overlapping section.
This solution overcomes the limits of the multi-fold interfolding machines of traditional type, i.e. that of cutting a web of paper into sheets and processing the sheets of a single length.
However, such machines require an area for arranging different modules and involve costs of each modular additional portion and, even if require a relatively quick production change time, they need in any case a maintenance stop.